factor out common PRNG reseed before privdrop

Add a call to RAND_poll() to ensure than more than pid+time gets
stirred into child processes states. Prompted by analysis from Jann
Horn at Project Zero. ok dtucker@

1 files changed, 23 insertions, 19 deletions

diff --git a/sshd.c b/sshd.c
index d7e812000..ce4a493ef 100644
--- a/sshd.c
+++ b/sshd.c
@@ -499,9 +499,29 @@ demote_sensitive_data(void)
 }
 
 static void
-privsep_preauth_child(void)
+reseed_prngs(void)
 {
         u_int32_t rnd[256];
+
+#ifdef WITH_OPENSSL
+        RAND_poll();
+#endif
+        arc4random_stir(); /* noop on recent arc4random() implementations */
+        arc4random_buf(rnd, sizeof(rnd)); /* let arc4random notice PID change */
+
+#ifdef WITH_OPENSSL
+        RAND_seed(rnd, sizeof(rnd));
+        /* give libcrypto a chance to notice the PID change */
+        if ((RAND_bytes((u_char *)rnd, 1)) != 1)
+                fatal("%s: RAND_bytes failed", __func__);
+#endif
+
+        explicit_bzero(rnd, sizeof(rnd));
+}
+
+static void
+privsep_preauth_child(void)
+{
         gid_t gidset[1];
 
         /* Enable challenge-response authentication for privilege separation */
@@ -513,14 +533,7 @@ privsep_preauth_child(void)
                 ssh_gssapi_prepare_supported_oids();
 #endif
 
-        arc4random_stir();
-        arc4random_buf(rnd, sizeof(rnd));
-#ifdef WITH_OPENSSL
-        RAND_seed(rnd, sizeof(rnd));
-        if ((RAND_bytes((u_char *)rnd, 1)) != 1)
-                fatal("%s: RAND_bytes failed", __func__);
-#endif
-        explicit_bzero(rnd, sizeof(rnd));
+        reseed_prngs();
 
         /* Demote the private keys to public keys. */
         demote_sensitive_data();
@@ -616,8 +629,6 @@ privsep_preauth(Authctxt *authctxt)
 static void
 privsep_postauth(Authctxt *authctxt)
 {
-        u_int32_t rnd[256];
-
 #ifdef DISABLE_FD_PASSING
         if (1) {
 #else
@@ -651,14 +662,7 @@ privsep_postauth(Authctxt *authctxt)
         /* Demote the private keys to public keys. */
         demote_sensitive_data();
 
-        arc4random_stir();
-        arc4random_buf(rnd, sizeof(rnd));
-#ifdef WITH_OPENSSL
-        RAND_seed(rnd, sizeof(rnd));
-        if ((RAND_bytes((u_char *)rnd, 1)) != 1)
-                fatal("%s: RAND_bytes failed", __func__);
-#endif
-        explicit_bzero(rnd, sizeof(rnd));
+        reseed_prngs();
 
         /* Drop privileges */
         do_setusercontext(authctxt->pw);